The present invention is directed to detecting and treating Alzheimer's disease, and more particularly to the detection and prevention of Alzheimer's disease at prodromal and early stages.
Alzheimer's disease (AD) is the most common neurodegenerative disorder affecting millions of elderly world-wide and desperately demands both specific prevention for future victims and effective therapies for those currently suffering. It results in memory loss, behavior and personality changes, and a decline in thinking abilities. It is believed that up to 4 million Americans suffer from AD. The disease usually begins after age 60, and the risk goes up with age. The number of people with AD doubles every 5 years beyond age 65. An estimated 35 million people or 13 percent of the total US population are now aged 65 or older and this percentage is expected to increase rapidly when the first baby boomers reach age 65. The estimated annual national cost of caring for AD patients is $100 billion (National Institute on Aging 2001). Amyloid plaques (SPs) and neurofibrillary tangles (NFTs) within the brain are the hallmark signs of AD. Unfortunately, the presence of these structures has been subject to confirmation only post mortem.
However, limited understanding of the pathogenesis, especially in prodromal and early stages, has largely hampered the continuing efforts in this regard. Although clinicians are increasingly making the diagnosis of mild cognitive impairment (MCI) commensurate evidence histologically and biochemically is lacking. Common in advanced AD brains are SPs and NFTs, the pathological hallmarks of AD (Reference 1). Closely associated with these pathologic changes are apparent signal transduction system aberrancies (References 2-3). In more detail, these disturbances have concentrated on various G-protein coupled receptors (GPCRs) (Reference 4) and their downstream effectors, such as phosphoinositide metabolism (Reference 5), protein kinase C activity (References 2 and 6) and calcium homeostasis (Reference 7). Several authors have pointed out that the locus of the signal transduction deficits appears to be at the receptor-G protein interface (References 4 and 8), where uncoupling of a GPCR at its C-terminus to its specific GTPase normally occurs, although specific responsible molecules remain to be identified.
GPCRs comprise one of the largest gene families in the human genome, and mediate a huge variety of cellular functions regulated by neurotransmitters, hormones, chemokines, and many other molecules. Timely uncoupling of GPCR signaling is crucial for maintaining appropriateness and integrity of the GPCR-mediated physiological functions. This uncoupling is primarily mediated by a much smaller gene family, currently numbering seven members, of GPCR kinases (GRKs) (References 9-10). The specificity for a few GRK members to regulate a huge numbers of GPCRs is controlled in an agonist-dependent manner. In another words, GRKs preferentially bind to and phosphorylate agonist-occupied GPCRs to uncouple receptor from corresponding G-protein, a process known as homologous desensitization (Reference 11). Based on structural similarities, seven known GRK members are classified into four subfamilies (GRK1, GRK2/3, GRK4/5/6 and GRK7), with GRK2/3 and GRK5/6 having ubiquitous distributions including brain (References 9-10). Dysregulation of GRK2, probably GRK5 as well, has been implicated in the pathogenesis of chronic heart failure (Reference 12), myocardial ischemia (Reference 13), and hypertension (Reference 14), etc. cardiovascular disorders, where the GRKs have been extensively studied (Reference 10). Failure to desensitize rhodopsin signaling by GRK1 (Reference 15) can lead to photoreceptor cell death, and is believed to contribute to retinitis pigmentosa (Reference 16). In addition, increased GRK2 levels have been associated with opiate addiction (Reference 17). Aside from these, however, roles of GRKs in many other pathological conditions potentially associated with GPCR deregulation, such as in AD, remain virtually unexplored.
Due to the membrane location of GPCRs, GRK's retention on the plasma membrane or in the cytosoal physically affects its access and binding to GPCRs. In resting cells, GRK4 subfamily members (including GRK4/5/6) are tightly associated with the plasma membrane (Reference 10), while GRK2 subfamily members (GRK2/3) are primarily cytosolic and translocate to the membrane when cells are stimulated by GPCR agonists (References 10 and 18). However, in active cells, subcellular localization of GRKs appears to be determined by the content and capacity of GRK-binding factors in membrane versus cytosol. Phospholipids, particularly phosphatidylinositol-4,5-biphosphate (PIP2), appear to be essential for GRKs to adhere to the membrane and bind GPCRs (Reference 19), while phosphatidylserine (PS) may also enhance GRK2 binding to GPCRs on the membrane (Reference 20). On the other hand, calcium/calmodulin and other calcium-binding proteins, as well as actin, actinin, etc. may contribute to sequester GRKs in the cytosol and inhibit binding of GRKs to GPCRs (References 11 and 21). In AD brains, significant membrane alterations (Reference 22), aberrant phosphoinositide metabolism (Reference 5), disrupted calcium homeostasis (Reference 7) and disorganized cytoskeleton proteins (Reference 23) could all influence the subcellular distribution of GRKs. In addition, increased β-amyloid (Aβ), a hydrophobic peptide central to AD pathogenesis, has been shown to decrease membrane PIP2 (Reference 24) and increase [Ca2+]i (Reference 25). Taken together, these findings have led us to investigate whether GRKs may contribute to the signal transduction system disturbances in AD brains. If yes, whether abnormal accumulation of Aβ might contribute to GRK dysregulation in the pathogenesis of AD. As a first attempt to answer these questions, we examined the expression and subcellular distribution of GRK2 and GRK5 in autopsied AD brains and in an early onset AD transgenic model, CRND8 mice. We also pursued further mechanistic studies in cultured murine microglial cells by investigating the impact of Aβ on GRK subcellular localization and regulation of GPCR signaling.
Currently there are no diagnostic methods available in the market that can detect AD accurately and reliably. The only accurate method now available is by postmortem examination of the brain.
Nymox, Inc. has developed a test called AlzheimAlert® that measures the amount of cerebrospinal fluid of a brain protein known as AD7C-NTP (neural thread protein) that is found in higher-than-normal concentrations in Alzheimer's patients. This urine test is considerably cheaper than any other test currently on the market, and the company claims over 90 percent accuracy. Even though the company received FDA clearance, it has experienced difficulty gaining industry acceptance (Medical Devicelink 1997).
FMG Innovations, Inc. recently released the Early Alert Alzheimer's Home Screening Kit. This self-administered test uses 12 scent strips to screen patients. According to the company, a decrease in the sense of smell is one of the early indicators of AD (FMG Innovations 2001).
Neurochem, Inc. and Nycomed Amersham Imaging announced an agreement to develop a product for the diagnosis of Alzheimer's disease. The companies will use Neurochem's proprietary molecule as a basis, and work together to design a system that will detect the presence of amyloid plaques in the brain. It is anticipated that the successful completion of the project will result in a product capable of accurately detecting early-stage Alzheimer's disease (Market News Publishing Inc. 2001).
Currently, there are no drugs available in the market to stop the progression of Alzheimer's disease. The drugs that are being marketed are agents for symptomatic treatment and/or to slow the progression of the disease.
Antipsychotic agents are often used to treat some of the symptoms experienced by patients with senile dementia brought on by AD. The leading drugs in this market are: tacrine (Cognex®) from Warner Lambert; donepezil (Aricept®) from Eisai (co-marketed by Pfizer), and galantamine HBr (Reminyl®), developed by Shire and to be marketed by Janssen Pharmaceutica Products, L.P. All three products can act as acetylcholinesterase inhibitors and are used to treat the memory/cognitive impairment that is often the earliest sign of AD.